Level Transmitters Information

level transmitters selection guideLevel transmitters provide an electrical output proportional to the level of a liquid or bulk solid in a confined space. They may provide an output upon reaching a specified value (point level) or repeatedly after each measurement (continuous level). Level transmitters measure media such as water, fuels, and viscous fluids, or dry media like powders and bulk solids.

 

Level transmitters are used in a wide variety of applications requiring level measurement within tanks or containers. The chemical, food and beverage, water treatment, material handling, and power industries often employ level transmitters.

 

A user must consider a level transmitter's measurement range and output compatibility when selecting a device. The Sensor Transmitters guide contains more information about these specifications.

 

Output

 

Level transmitters transmit a particular output signal that is ideally compatible with a corresponding sensor and meter/indicator in a given application. Common outputs include:

 

  • Current loops are among the most common sensor outputs. Disciplines such as process control extensively use 4-20 mA outputs, as they feature long-range transmission capabilities. Current signals are not affected by voltage drop within system wiring, so that current may also power measurement devices. Digital signals using Highway Addressable Remote Transducer (HART) protocol may supplement analog current loops.
  • Fieldbus protocols are digital communications standards that have replaced current loops in many applications. Examples include FOUNDATION Fieldbus, Profibus, and Modbus.
  • Frequency signals include sine waves, pulse trains, or AM/FM outputs.

Liquid Measurement Types

 

Liquid level measurement employs four main technology types as described in the table below. For a more complete description of both liquid and solid measurement techniques, please visit the Level Sensors guide.

 

Type

Description

Advantages

Disadvantages

Capacitance

Measures current conductivity proportional to level. Value is a function of fluid’s dielectric constant.

Easy to install; ideal for use with slurries and other liquids containing solid particles.

Difficult to use with non-metallic tanks; dielectric constant limits measuring range; probes must be relatively long.

 level transmitters selection guide

Pressure

Measures hydrostatic pressure as a function of height/level.

Ideal for foamy or agitated media.

Limited to low-viscosity liquids and non-pressurized tanks.

 level transmitters selection guide

Radar

Uses time domain reflectometry (TDR) to transmit and receive reflected signals to determine level.

Easy to install; ideal for both liquids and solids; unaffected by contaminants.

Probe must be as long as entire sensing range; expensive.

 level transmitters selection guide

Ultrasonic

Measures time between emission and reflection of acoustic signals to determine level.

Non-contact measurement; ideal for viscous fluids; full measurement range is typically programmable.

Not suitable for steamy or foamy liquids or highly pressurized tanks.

 level transmitters selection guide

 

Image credit: Instrupedia |  Sigma Hellas | Chipkin | Sigma Hellas

 

References

 

Sensors Online—A dozen ways to measure fluid level

 

Image credits:

OMEGA Engineering

 

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